The present invention relates to a facsimile compression system, in particular, relates to a facsimile compression system which transmits the picture information in an analog form.
In order to reduce the transmission time of a picture in a facsimile system, many facsimle compression system in which a large white area in a transmission picture is not scanned but only areas having both the white and black cells are scanned, have been proposed. Some of them are run length coding system (a first order run length coding, a second order run length coding), a relative address coding system, and a white picture skip system. In a run length coding system, the continuous length of white cells or black cells is coded in a binary form and then the code is transmitted instead of a scanned signal. FIG. 1(A) shows the example of the run length system, in which the symbol ".quadrature." is "1" for black cells and is "0" for white cells, and the symbol "xx" indicates the binary code showing the continuous length of white or black cells. For instance, when three black cells continue, the scanned signal is coded to "111". In a relative address coding system, each cell is coded considering the comparison with the corresponding cell on the nearest preceeding scanning line. In a white picture skip system, each scanning line is divided into a plurality of sections, and when a specific section has no black cell, that is to say, all the cells in the section are white, that section is not transmitted, and is skipped. In order to indicate the skipped sections, an indicator block is inserted preceeding the picture sections as shown in FIG. 1(B). In FIG. 1(B), SYC is the synchronization code which is placed at the head of each scanning line. IND is the indicator block showing whether or not each picture section is skipped. In the example of FIG. 1(B), the first two bits in the indicator block are "0", so the first two picture sections are skipped and are not transmitted. The third bit of the indicator block is "1", so the third picture section "P.sub.3 " is not skipped and is transmitted in an anolog form. The fourth section is skipped, the fifth and sixth sections are transmitted.
However, in a prior facsimile compression system described above, all of the information, or at least a part of it, is transmitted in a digital form since the compressed signal is coded in a binary form. Accordingly, when a facsimile signal is transmitted through a telephone line, a digital modem (modulator and demodulator) must be installed. Further, a transmission error of only a single bit would cause a large error on the specific scanning line, or even a plurality of adjacent scanning lines at a receiving station, as the signal is transmitted in a coded form. It should be appreciated that if a signal is transmitted in an analog form, a single error would cause a small spot of noise at a receiving station. Accordingly, a high quality transmission line must be utilized for the transmission of compressed facsimile signals, thus the transmission modem must be of high quality assuring an error rate of 10.sup.-4 or 10.sup.-5, which is almost the same as for the data transmission system. Therefore, when a high transmission speed like 4800 baud is utilized, the transmission cost becomes considerably high. In the case of a white picture skip system, an error in an indicator block will completely disturb the picture quality and the system can not even be synchronized. Further, although we intend to use an analog type modem instead of digital type modem for the transmission of a compressed facsimile signal, the transmission speed must be considerably lowered for assuring a small transmission error. Therefore, an analog type modem is useless for a prior compressed facsimile signal.